In recent years, development of fuel cells that offer high power generation efficiency and emit no carbon dioxide has been promoted from the viewpoint of preserving the global environment. A fuel cell is a device that generates power by causing hydrogen and oxygen to react with each other. A basic structure of a fuel cell resembles a sandwich and is constituted by an electrolyte membrane (i.e., ion exchange membrane), two electrodes (i.e., a fuel electrode and an air electrode), a diffusion layer for diffusing hydrogen and oxygen (air), and two separators. Phosphoric-acid fuel cells, molten carbonate fuel cells, solid-oxide fuel cells, alkaline fuel cells, proton-exchange membrane fuel cells, and the like have been developed in accordance with the type of electrolyte used.
Of these fuel cells, proton-exchange membrane fuel cells in particular have following advantages over molten carbonate fuel cells, phosphoric-acid fuel cells, and the like:
(a) Operation temperature is significantly low, i.e., about 80° C.
(b) Weight- and size-reduction of the fuel cell main body is possible.
(c) The time taken for start-up is short and fuel efficiency and output density are high.
Accordingly, proton-exchange membrane fuel cells are one of the most prospective fuel cells for onboard power supplies for electric vehicles and portable and compact dispersed power systems for household use (stationary type compact electric generator).
A proton-exchange membrane fuel cell is based on the principle of extracting power from hydrogen and oxygen through a polymer membrane has a structure shown in FIG. 1, in which a membrane-electrode assembly 1 is sandwiched by gas diffusion layers 2 and 3 such as carbon cloths and separators 4 and 5 to form a single constitutional element (also known as a single cell). Electromotive force is generated between the separators 4 and 5.
The membrane-electrode assembly 1 is also known as MEA and is made by integrating a polymer membrane and an electrode material such as carbon black supporting a platinum catalyst, the electrode material being provided on front and back surfaces of the polymer membrane. The thickness of the membrane-electrode assembly 1 is several ten to several hundred micrometers. The gas diffusion layers 2 and 3 are frequently integrated with the membrane-electrode assembly 1.
When proton-exchange membrane fuel cells are applied to the usages described above, several ten to several hundred single cells described above are connected in series to form a fuel cell stack, and the fuel cell stack is used.
The separators 4 and 5 typically have the following functions:
(A) a function of a separator that separates between single cells;
(B) a function of a conductor that carries electrons generated;
(C) a function of a channel for oxygen (air) and hydrogen (air channels 6 and hydrogen channels 7 in FIG. 1); and
(D) a function of a discharge channel for discharging water and gas generated (air channels 6 and hydrogen channels 7 also serve as this discharge channel).
In order to use a proton-exchange membrane fuel cell in practical application, separators having good durability and conductivity must be used.
The durability expected is about 5000 hours for fuel cells for electric vehicles and about 4000 hours for stationary type electric generators used as compact dispersed power systems for household use and the like.
Proton-exchange membrane fuel cells that have been put to practice use carbon materials as separators. However, since carbon separators are susceptible to fracture upon impact, not only the size-reduction is difficult but also the process cost for forming channels is high. In particular, the cost problem has been the largest impediment for spread of fuel cells.
In response, attempts have been made to use a metal material, in particular, stainless steel, instead of carbon materials as the material for separators.
As discussed earlier, separators have a function of a conductor for carrying electrons generated and must have conductivity. With respect to the conductivity in the cases where stainless steel is used as separators, the contact resistance between the separators and gas diffusion layers becomes dominant. Thus, a technique for reducing the contact resistance has been pursued.
For example, PTL 1 discloses stainless steel including 1011 laves phases having a grain diameter of 0.3 μm or more in a surface per square meter.